Geodimeter EDM's of this generation were exceptionally compact, powerful & provided results almost immediately without having to attenuate the infrared signal. Useful not only for surveying & setting-out new construction but equally suited to inshore hydrographic surveying where the wide measuring beam, range & the time taken for a position fix ( 0.4 secs of time) gave the 100 series advantage over other distance meters on the market. The distance meter was generic in the sense that it could be mounted on almost any manufacturers' instruments. Thus not only was performance an attraction but for the surveyor it could be mounted on their instrument of choice.In 1981 the GDM 122 succeded the GDM 120 & introduced the Tracklight & Unicom additions. The Tracklight was similar to the HP Guidelight released in 1986.